I took my neighbor for his first mountain bike ride today. We rode little sac and it was a lot of fun. He's a college student in his early twenties and is in pretty good shape as a competitive runner. It's actually the first time I've taken someone for their maiden voyage where they didn't completely boink. We rode for about an hour and half and if it hadn't started raining we could have gone longer. He did very well on the uphills and went understandably slow on the downhills. Really the only time he had to come off the bike was when i didn't give him a headsup about a steep incline we were approaching. Speed comes with experience and it's never a good idea to ride beyond your comfort level.
By the end of the ride his hands were starting to hurt (I think he had a death grip on the bar during the descents) but he had fun and said that mtbing wasn't at all what he expected. I had him ride my rockhopper because he has a hybrid bike and I'm sure it would have handled it but off road riding is so much more enjoyable when you're on a quality bike. I've seen people on the trails on walmart bikes and I think it's good they're out there but I know they would have a better experience on a lighter, better handling bike
Anyway, I hope he wants to get out there again. I usually ride alone and it was cool to have someone to share the experience with.